A055520 Numbers k such that 30*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 3, 4, 5, 6, 7, 17, 39, 49, 59, 77, 100, 150, 318, 381, 783, 1731, 1917, 8854, 11244, 11959, 12129, 18532, 22717, 23364, 24252, 24548, 25323, 30177, 53717, 380975, 424860

Offset: 1

Author

Eric W. Weisstein

Keywords

• hard
• nonn

Comments

Also numbers k such that (10^(k+1)-7)/3 is prime.

References

• Martin Gardner, Strong Laws of Small Primes, in The Last Recreations, p. 194 (1997).

Links

• Makoto Kamada, Prime numbers of the form 33...331.
• Eric Weisstein's World of Mathematics, 3
• Index entries for primes involving repunits

Crossrefs

Cf. A055557.

Indices of A033175 that are primes. Cf. A051200, A055557.

Programs

Mathematica

Do[ If[ PrimeQ[30*(10^n - 1)/9 + 1], Print[n]], {n, 0, 50410}]

Formula

a(n) = A055557(n) - 1. - Robert Price, Jan 30 2015

Extensions

a(32)-a(33) from Leonid Durman, Jan 09-10 2012